Indirectly heated cathode



Sept 17, 1935. J H. JQNKER 2,014,34?

INDIRECTLY HEATED CATHODE Filed July 20, 1954 INVENTOR JOHAN L.H. JONKERelectron emitting material.

Patented Sept. 17, 1935 UNITED STAT A i'N'l' OFFICE INDIRECTLY HEATEDCATHODE Application July 20, 1934, Serial No. 736,211 In GermanySeptember 7, 1933 5 Claims.

This invention relates to an indirectly heated cathode for electricdischarge tubes comprising at least a heating member and a cathodeproper consisting of a metal carrier body coated with These cathodes mayconsist, for instance, of a heating member surrounded by and, ifdesired, insulated from the carrier body which carries the emittinglayer.

With such cathodes the electron emitting material, such as a suspensionof an alkali or alkaline earth compound, is sometimes applied byimmersion to the surface of the carrier body. However, it is alsopossible to apply such a compound, or some other electron emittingmaterial, by spraying, or by disintegration of the compound on thesurface of the carrier body.

It has previously been known that it is important to reduce as much aspossible the heat radiated by the carrier body and by the electronemitting material, which heat is consequently derived from the heatingmember, in order that the consumption of energy for heating the cathodeto the required temperature be as small as possible. This might be done,as has already been proposed,

' by reducing as much as possible the thickness of the emissive layerapplied to the carrier body. Furthermore it is possible, as has alreadybeen suggested, to reduce the loss of energy due to radiation of thecarrier body and of the emissive layer by using a carrier body which ismade of material with a very small heat radiating capacity, and on whichthe emitting material is applied in the form of a thin layer.

I have now found that the desired reduction in the amount of heatradiated from the cathode can be obtained in a very suitable manner byusing an indirectly heated cathode constructed according to theinvention. With this cathode, which consists of a heating member and ametal carrier body, the electron emissive layer or coating is applied insuch a manner that it is discontinuous and parts of the surface of thecarrier body are left uncovered or free between various parts of thisemissive layer.

By this invention the result is achieved that the heat radiation of thecathode body itself, together with that of the electron emittingmaterial, can be materially reduced. In fact, more particularly whenusing thin layers or coatings of electron emitting material, the sum ofthe heat radiation of both the cathode surface and the emitting layersurface is always involved, since the heat radiation of the cathodesurface traverses the surface of the layer of emitting material. Bymeans of the invention it is possible to reduce to zero the heatradiation of the emitting layer over part of the surface of the cathode.It will be appreciated that in this manner an improvement of theproperties of such cathodes can be obtained. 5

In order to avoid affecting the electrical properties of discharge tubesequipped with cathodes made according to the invention, it has beenfound to be very advantageous to make the distances between the parts ofthe coating of emit- 10 ting material small relatively to the distancebetween the cathode and the first grid, for example, of the order ofone-tenth of this distance. In the usual discharge tubes with thedistance between the cathode and the grid about 0.5 mm., it has beenfound to be advantageous to make the distance between the discrete partsof the emissive layer about 0.05 mm.

It has turned out that the invention lends itself more particularly foruse with indirectly heated cathodes in which the carrier body for theemitting material consists entirely or partly of a metal of less heatradiating capacity than that of nickel, and preferably a metal of thecopper class, such as copper or silver. Even if, when using thesemetals, with which consequently the substratum of the emitting materialhas a very small heat radiating capacity, this emitting material isapplied in the form of extremely thin layers, the heat radiation of theemissive layer plays in spite thereof such a part that when using thepresent invention a considerable improvement is obtained also with suchcathodes.

The object of the invention may advantageously be attained by applyingthe emitting layer by spraying or by disintegration of suitablecompounds. In this manner it is readily possible to make the electronemissive layer on the carrier body in such a way that it isdiscontinuous and that part of the surface of the carrier body remainsuncovered between the discrete parts of the layer.

In the accompanying drawing, which shows as one example a cathode madeaccording to the invention and manufactured according to the followingmethod given by way of example, Figure 1 is a longitudinal section ofthe cathode, and Figure 2 a plan view of part of the cathode surface.

In the drawing, in which the cathode made according to the invention isrepresented very diagrammatically, I is the heating member of anindirectly heated cathode of which the carrier body consists of a metaltube 2. An insulator 3 is provided between the heater 1 and the carrierbody 2.

On the outside of the carrier body 2 an electron emissive layer orcoating 4 is applied in such a way that it is discrete or discontinuousand parts of the underlying metal surface between the many parts of thiscoating are left uncovered. The distance between the separated ordiscrete parts of the coating 4 is about 0.05 mm. The cathode issurrounded by a grid 5, which is spaced at a distance of about 0.5 mm.from the cathode, and which in turn is surrounded by an anode or plate6.

The cathode referred to may be made as follows: Several cathodes whichhave not yet been provided with electron emitting material areintroduced into a spraying, or coating mill. A coating consisting, forinstance, of barium and strontium carbonate is applied to these cathodesin some well known manner, such as spraying. In order to insure a gooddistribution over the cathode surface the spraying is done with low airpressure, the solution or suspension containing a comparatively largequantity of solid material with respect to the liquid. The aperturedischarge opening of the spray gun is comparatively large, for instance,1.5 mm. so that a definite quantity of perfectly distributed barium andstrontium carbonates can be readily applied on the cathode.

After the spraying operation, by which the coating is applied in such away to the cathode that part of the cathode surface remains free anduncovered between the various parts of the emissive coating, thecathodes are removed from the spraying mill, and further finished andtreated in the usual way.

It will be appreciated that the manufacture of the cathode according tothe invention is not limited to the embodiment referred to above, butalso includes many other methods of manufacturing such a cathode.

What I claim as new is:-

1. An indirectly heated cathode for electron discharge tubes consistingof a heating member, a metal carrier adjacent said heating memher, and adiscontinuous electron emitting coating on said carrier composed ofdiscrete parts distributed substantially uniformly over the surface ofsaid carrier and spaced apart on said surface a distance of about 0.05mm.

2. An indirectly heated cathode for electron discharge tubes consistingof a heating member and a carrier of a metal of the copper class mountedadjacent said heating member and coated with material of high electronemissivity distributed on the surface of said carrier to form adiscontinuous coating of discrete parts spaced about 0.05 mm. to leaveuncovered part of the surface of said carrier.

3. An indirectly heated cathode for electron discharge tubes consistingof a heating member, a carrier adjacent said heating member of copper,and a discontinuous electron emitting coating composed of discrete partseach containing barium and strontium compounds and distributed on saidcarrier to be spaced about 0.05 mm. and to leave uncovered part of thecarrier surface between the discrete parts of the electron emittingcoating.

4. An indirectly heated cathode for electron discharge tubes consistingof a heating member, and a metal carrier adjacent said heating membercoated with alkaline earth metal compounds of high electron emissivitydistributed on the carrier body to form a discontinuous electronemitting coating consisting of discrete spots of said compounds spacedabout 0.05 mm. with the carrier surface between spots free of saidcompounds.

5. An electron discharge device comprising an indirectly heated cathodeconsisting of a heating member and a metal carrier adjacent said heatingmember, a grid electrode surrounding and spaced away from said carrier,and a discontinuous coating on said carrier of material of high electronemissivity distributed over the surface of said carrier in discreteparts with the distances between said discrete parts about one tenth ofthe distance between said carrier and said grid electrode.

J OI-IAN LODEWIJ K HENDRIK J ONKER.

